A problem concerned with turbine motors is the difficulty to accomplish a simple yet reliable overspeed preventing safety device by which the rotor is prevented from reaching hazardous high speed levels where damage to equipment and operator injuries may be caused. Normally a safe and suitable speed is provided by a speed governor of any suitable type, but in case of a speed governor malfunction there has to be an independently operating overspeed preventing device fitted to protect the operator and other equipment from damage and injury. Since turbine motors operate at very high speed levels it is very difficult to design a reliable and safely operating overspeed preventing device of the type comprising a mechanical speed responsive actuator and a pressure medium shut-off valve. Such an actuator/valve arrangement is also rather complex in design and costly to manufacture as it contains many parts which require expensive machining.